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```Creating Living Hinges

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Creating Living Hinges

Written By: Kevin Gunn

SUMMARY

I have a particular interest in discovering new ways to push the limits of material fabrication
with my laser cutter. In this guide I'm going to give you an overview of living hinges that will
allow you to make bends (and springs!) in a single piece of rigid material. Wood and acrylic
are particularly amenable to this technique, and the result has an artistic aspect that's pretty
cool as well!

More importantly, I'm going to try to add a bit of theory to this so that you can extend these
techniques in new and better ways.

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Creating Living Hinges

Step 1 — Creating Living Hinges

• What does it mean for a material to be rigid in the sense I'm using the term? A rigid
material is one that has very limited ability to bend over a given length of that material

• Acrylic is RIGID. Very, very little flexibility, but it's a beautiful material. So how do we get it
to bend?

• The acrylic sample that I cut here has the ability to fold easily from fully open (flat) all the
way to fully closed (180 bend) in either direction. It also can act as a spring along its
length.

• How does this work? Head to the next step for the physics and geometry of it.

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Creating Living Hinges

Step 2

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• I use Google Sketchup for my
design work. Here's a screen
capture of the simplest method for
making living hinges: a zigzag.

• Remember how I defined "rigid" in
Step 1 ? Acrylic will only bend
naturally VERY slightly per unit
length before it breaks (wear safety
glasses if you test this).

• Here the acrylic sample is 120mm
long, 40mm wide and 5.5 mm thick.
Maximum possible bend over the
length is just a couple or three
degrees without the living hinge.

• What I'm doing here is effectively
EXTENDING THE LENGTH of the
acrylic to make a big bend in a
small space. The "bend zone" in
the image is only 39mm, but the
zigzag means the bend is occurring
over about 9 zigs of 40mm each
(360 mm). In effect, the hinge is 3
times the length of the total piece.

• Additionally, the 5.5 mm acrylic is
now THINNER in the hinge zone
since the zigzags are only 2mm
wide. This also increases the
amount of bend that is possible.

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Creating Living Hinges

Step 3

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• The problem with this design is that
it is FRAGILE. Notice that
throughout the hinge area that the
amount of acrylic connecting the
zigzags to each other is only 2mm
thick.

• Also, that 2mm of acrylic only
exists at the top or the bottom of
each zigzag -- that's about as
fragile a place as they could
possibly be!

• Finally, this design (while VERY
flexible and easy to understand)
allows a lot of rotation in directions
outside of the hinge axis (i.e. you
can twist it easily).

• So... can we improve on this
design now that we understand why
it works? Of COURSE we can!

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Creating Living Hinges

Step 4

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• By essentially overlaying two
zigzags in our design, we can
maintain a high degree of flexibility
in our hinge.

• The extra connections this creates
within the material greatly enhance
the strength of the object.

• This also reduces the ease with
which the material can twist and
stretch (and that may or may not
be a good thing depending on your
goals).

• PROBLEM: One out of two zigs
have TWO connections, but the
other half have ONE. That means
we STILL have weak spots.

• SOLUTION: See how a pattern of
offset gaps is emerging? Let's take
that one step further with a "Triple
Zig" (next step).

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Creating Living Hinges

Step 5

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Creating Living Hinges

• Finally, this staggered layout which
I call a "Triple Zig" is the strongest
variant I've tried so far. I've seen
other folks using similar cuts as
well so I'm probably not the first to
do it this way, though I haven't see
others doing exactly this cut in
acrylic yet. Acrylic is a bit less
forgiving than wood, so I 've settled
on the 2mm gap / 2mm zig width as
working best with 5.5 mm (1/4-
inch) acrylic.

• Notice that each "zig" has TWO
connections to its neighbors. This
almost eliminates twist.

• @solarbotics has had some luck
with slits in acrylic and @9600 and
@talldarknweirdo have been
working in this area as well. I think
we were all inspired by the work in
wood done by snijlab here:
http://www.snijlab.nl/?page_id=358

• This pattern has good strength,
good flexibility and low twist. It also
looks great! I'm hoping to create
some electronics enclosures with
this technique in the near future.

• I hope you found this technique and
explore and build on it and share
back what you learn with the Maker
Community. Open Sourcing
knowledge makes the whole world
richer. Thanks!

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Creating Living Hinges

This document was last generated on 201 2-1 0-31 1 1 :31 :48 PM.

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